At the Thames Barrier

Britain’s south-eastern coastline is low-lying, and faces relatively shallow waters. This makes London naturally vulnerable to the surges created when high tides coincide with North Sea storms, and shove water back up the Thames. We have records of the city being flooded since the Anglo-Saxon era. In 1236 there were boats rowing through the Palace of Westminster; in 1928 the Tate Gallery was drenched in mud. After a surge in 1953 killed hundreds of people across the Thames Estuary, the government commissioned a report on what to do from the mathematician and cosmologist Hermann Bondi.

The result was the Thames Barrier at Woolwich, operational since 1983. It consists of nine piers spaced across the river, with ten gates between them (and either bank). The design of the gates – their cross-section is a segment of a circle, like a narrow capital D – came to the engineer Charles Draper when he was using the taps on his gas cooker. When the gates are closed, the curved edges face downstream, towards the approaching sea. Wheels on the sides of the piers rotate the shields into position. The four gates closest to the banks are suspended above the water and lowered into it; the other six, in the centre of the Thames, are kept in curved recesses on the riverbed, and lifted into place; their ability to disappear keeps the river clear for traffic. These ‘rising sector gates’ vary a little in size and weight, but the four largest are 20 metres high, span 61.5 metres between piers, and weigh 3700 tonnes each.

The whole Barrier is half a kilometre long, and it takes an hour and a half to close. The gates are shut in pairs, from the outside to the centre; they form a wall of steel that holds the sea out of the city. The Environment Agency decides when to close it, usually when the Met Office has warned of weather conditions that would raise the Thames in central London above 4.87 metres. The potential calamity can vary: there might be an imminent storm surge, or the river might be swollen after heavy rain and scheduled to hit a rising tide. Once closed, the Barrier remains impassable until the waters on either side have levelled out, several hours later. In 34 years, there have been 178 closures. Central London hasn’t seen a single tidal flood.

The Barrier was designed to withstand a ‘1 in 1000 year event’, i.e. a flooding event with a 0.1 per cent chance of occurring in any given year. The original expectation was that it could last beyond 2030, meaning it should be late-middle-aged by now. But its designers worked on the assumption that sea levels would go up by eight millimetres a year, and so far we have seen less than half that rise. The Environment Agency admits that it can’t last for ever – ‘The Thames Barrier is a mechanical structure built in a salt water environment. In time it will degrade, regardless of sea level rise or climate change’ – but it's expected to keep working until 2070. The current plan is to replace it with a new, bigger, more complicated barrier, twenty miles downstream at Long Reach. Nobody has yet taken a stab at the cost of that.

But there is one great unknown factor: climate change. In 2013, the IPCC predicted that the oceans would rise, on global average, by 62 centimetres this century. On the other hand, according to a recent study in Environmental Research Letters, the worst-case scenario might be more like three metres. Add that to the high-tide depth of the Thames at Woolwich of around fifteen metres, budget a few more for a storm surge, and the Barrier, with its twenty-metre gates, would be struggling. Such a colossal rise is very unlikely, so nobody is rushing to commit millions from the public purse; another can to kick down the road. But more than 1.3 million people live in the Thames Estuary, where there’s £275 billion of property; in London, the numbers rise to more than 8 million and nearly £1.5 trillion.

The Barrier isn’t something we are encouraged to contemplate. When I went to look at it a few weeks ago, the first thing that struck me was its lack of interest in attracting visitors. It isn’t hard to get to, but it isn’t heavily signposted or advertised, either. For two hours, I was the only person there. The weather was overcast, and not warm. The Thames looked drab, as it often does.

The structure is fundamental to the lives of millions of Londoners, which may be the reason very few of them want to look at it. ‘The real job of the Barrier,’ a character in Iain Sinclair’s novel Downriver mutters to himself, ‘is to retain the sleep of the city; not to let our dreams – the most precious of all resources – escape.’

Comments

I'd be interested to know why the original designers anticipated that (so far, excessive) annual sea-level rise. Was it an early idea about potential global-warming issues? Or some naturally occurring cyclical thing? Or what?

Rust in the muddy estuary, steel soaked in salt water, the "mechanical structure built in a salt water environment" (sea) - nothing lasts, not even 2017.

At one time, when the Thames was a tributary of the Rhine and Britain was joined to mainland Europe by the delightful tropical savannah of Doggerland with its happy tribes well fed on mastodon, then there was no geographical separation to worry about and the rumble of glaciers was a distant threat - but they, too, came, conquered and passed.